Dual emission organic light emitting display device and method of driving the same

ABSTRACT

A dual emission organic light emitting display device and method of driving the same. The display device includes a pixel driver and an organic light emitting diode that can display different images on a top surface and a bottom surface and/or a same image on both the top and bottom surfaces. The display device includes a top/bottom selector that receives a driving current from the pixel driver and selectively supplies the driving current to a top organic light emitting diode or a bottom organic light emitting diode. The top/bottom selector includes transistors, which are connected between the pixel driver and the organic light emitting diode and select a top emission operation or a bottom emission operation. Here, the circuit configuration of the pixel driver is reduced so that the dual emission organic light emitting display device can be improved in terms of a layout, an interconnection, and an aperture ratio.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2005-0078758, filed Aug. 26, 2005, the entire contentof which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dual emission organic light emittingdisplay device, and more particularly, to a dual emission organic lightemitting display device that can display different images on a topsurface and a bottom surface and/or a same image on the top surface andthe bottom surface using a single pixel driver, and a method of drivingthe same.

2. Description of the Related Art

An organic light emitting display device is an emissive display in whichlight is created by electrically exciting organic compounds. That is,the organic light emitting display device includes an organic lightemitting diode having an organic emission layer interposed between ananode and a cathode. Thus, holes supplied from the anode combine withelectrons supplied from the cathode in the organic emission layer toform hole-electron pairs, i.e., excitons. When the excitons transitionfrom an exited state to a ground state, energy is generated so that theorganic light emitting diode emits light.

In general, the organic light emitting display device can be classifiedinto a passive matrix type and an active matrix type depending on amethod of driving N×M pixels that are arranged in a matrix shape.

The passive matrix type can be fabricated by arranging anodes andcathodes in a matrix shape on a display region.

By contrast, in the active matrix type, a thin film transistor (TFT) isdisposed on each pixel of a display region. Thus, a constant amount ofcurrent can be supplied to an organic light emitting diode irrespectiveof the number of pixels so that the active matrix type can emit lightwith uniform luminance. Also, since the active matrix type consumes lesspower than the passive matrix type, it can be applied to high-resolutionlarge-sized display devices.

Also, the organic light emitting display device can be categorized intoa bottom-emitting type and a top-emitting type depending on thedirection in which light generated from an organic emission layer isemitted.

In the bottom-emitting type, light generated from an organic emissionlayer is emitted toward a glass substrate. The bottom-emitting typeincludes a reflective layer (for a cathode) disposed on the organicemission layer and a transparent electrode (for an anode) disposed underthe organic emission layer. Here, in the active matrix type, since aportion where a TFT is formed does not transmit light, the lightemitting area can be reduced.

By contrast, in the top-emitting type, a transparent electrode (for acathode) is disposed on an organic emission layer, and a reflectivelayer (for an anode) is disposed under the organic emission layer. Here,since light generated from the organic emission layer is emitted in adirection opposite to the substrate, the area or portion that thattransmits light can increase, thereby increasing the luminance of theorganic light emitting display device.

A dual emission organic light emitting display device is an organiclight emitting display device in which a top-emitting type and abottom-emitting type are formed on a single substrate.

Typically, the dual emission organic light emitting display devices canbe categorized into first, second, and third types.

The first type employs a transmissive light emitting device structure sothat the same image is displayed on both surfaces. However, since thisfirst type displays the same image on both surfaces, it can only beapplied to a few number of applications.

The second type includes two bottom-emitting substrates that are broughtinto contact with each other, so that different images are displayed onboth surfaces. However, in the second type, although different imagescan be displayed on both surfaces, its production cost is high(doubled), and it is difficult to encapsulate such a device, therebylowering its reliability.

In the third type, a single substrate is divided into a top emissionregion and a bottom emission region so that the top emission region andthe bottom emission region can be separately driven. However, the thirdtype allows an image to be only partially displayed on both surfaces.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a dual emission organiclight emitting display device that can display different images on a topsurface and a bottom surface and/or the same image on the top surfaceand the bottom surface using a single pixel driver, and a method ofdriving the same.

In an exemplary embodiment of the present invention, a dual emissionorganic light emitting display device includes: a pixel driver adaptedto receive a top emission data signal for a first sub-frame of oneframe, to receive a bottom emission data signal for a second sub-frameof the one frame, and to generate a top driving current and a bottomdriving current; a top/bottom selector adapted to receive the topdriving current and the bottom driving current from the pixel driver andto selectively supply the top driving current and the bottom drivingcurrent in response to a top emission control signal and a bottomemission control signal; and a dual emission organic light emittingdiode adapted to receive one of the top driving current or the bottomdriving current, to emit light through a top surface for the firstsub-frame, and to emit light through a bottom surface for the secondsub-frame.

In another exemplary embodiment of the present invention, a dualemission organic light emitting display device includes: a pixel driveradapted to generate a driving current in response to a data signal; atop/bottom selector adapted to receive the driving current from thepixel driver and to selectively supply the driving current in responseto a top/bottom selection signal; and a dual emission organic lightemitting diode adapted to selectively receive the driving current fromthe top/bottom selector and to emit light through one of a top surfaceor a bottom surface.

In yet another exemplary embodiment of the present invention, a dualemission organic light emitting display device includes: a top organiclight emitting diode having a top first electrode formed with a topreflective layer, an organic layer, and a second electrode, the organiclayer being stacked between the top first electrode and the secondelectrode; a bottom organic light emitting diode having a bottom firstelectrode, the organic layer, and a second electrode formed with abottom reflective layer, the organic layer being stacked also betweenthe bottom first electrode and the second electrode; and a pixel driverelectrically connected to both the top first electrode and the bottomfirst electrode and adapted to selectively supply a driving current tothe top first electrode and the bottom first electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, together with the specification, illustrateexemplary embodiments of the present invention, and, together with thedescription, serve to explain the principles of the present invention.

FIG. 1 is a cross-sectional view illustrating a process of fabricatingred (R), green (G), and blue (B) organic light emitting diodes (OLEDs)of a dual emission organic light emitting display device according to anembodiment of the present invention;

FIG. 2 is a circuit diagram of R, G, and B pixel circuits of the dualemission organic light emitting display device of FIG. 1;

FIG. 3 is a timing diagram illustrating a method of driving the pixelcircuit of FIG. 2;

FIG. 4 is a circuit diagram of R, G, and B pixel circuits of a dualemission organic light emitting display device according to anotherembodiment of the present invention;

FIG. 5 is a timing diagram illustrating a first method of driving thepixel circuit of FIG. 4; and

FIG. 6 is a timing diagram illustrating a second method of driving thepixel circuit of FIG. 4.

DETAILED DESCRIPTION

In the following detailed description, only certain exemplaryembodiments of the present invention are shown and described, by way ofillustration. As those skilled in the art would recognize, the describedexemplary embodiments may be modified in various ways, all withoutdeparting from the spirit or scope of the present invention.Accordingly, the drawings and description are to be regarded asillustrative in nature, and not restrictive.

Embodiment 1

FIG. 1 is a cross-sectional view illustrating a process of fabricatingred (R), green (G), and blue (B) organic light emitting diodes (OLEDs)of a dual emission organic light emitting display device according to anembodiment of the present invention.

Here, only a process of fabricating an R OLED will be described indetail for ease of explanation, and it should be understood by thoseskilled in the art that G and B OLEDs can be fabricated withsubstantially the same process.

Referring to FIG. 1, the R OLED is a dual emission R OLED and is dividedinto a top R emission region and a bottom R emission region.

The dual emission R OLED includes a glass substrate 10 on which thinfilm transistors (TFTs) (e.g., as show in FIG. 2) are formed to generatea driving current. The TFTs supply the driving current to the R OLEDsuch that the dual emission R OLED emits light. The TFTs include asemiconductor layer formed by a low temperature polysilicon (LTPS)process. Also, the TFTs are formed in the top R emission region.

A planarization layer 20 is formed on the entire surface of the glasssubstrate 10, and a top reflective layer 25 is patterned on theplanarization layer 20 formed in the top R emission region. The topreflective layer 25 is formed of Al(Nd) or Ag, preferably, Al(Nd). Thetop reflective layer 25 is formed by a sputtering process or an ionplating process. The formation of the top reflective layer 25 includesdepositing, for example, an Al(Nd) layer, forming a photoresist (PR)pattern by a photolithography process, and selectively removing theAl(Nd) layer by an etching process using the PR pattern as an etch mask.

Thereafter, a top first electrode 30 is patterned on the top reflectivelayer 25 in the top R emission region, and a bottom first electrode 31is patterned on the planarization layer 20 in the bottom R emissionregion. When the top first electrode 30 and the bottom first electrode31 are anodes, they may be transparent electrodes formed of a materialhaving a high work function, such as indium tin oxide (ITO) or indiumzinc oxide (IZO). When the top first electrode 30 and the bottom firstelectrode 31 are cathodes, they may be transmissive electrodes formed ofa conductive metal having a low work function, e.g., a conductivematerial selected from the group consisting of Mg, Ca, Al, Ag, andalloys thereof.

In the dual emission organic light emitting display device of thepresent embodiment, since different images are displayed on bothsurfaces using only one pixel driver (e.g., as shown in FIG. 2), the topfirst electrode 30 is electrically connected to a source/drain (e.g., asshown in FIG. 2) of a top emission transistor through a contact hole,and the bottom first electrode 31 is electrically connected to asource/drain (e.g., as shown in FIG. 2) of a bottom emission transistorthrough another contact hole. The one pixel driver includes a pluralityof TFTs and is formed (e.g., the TFTs are formed) under the topreflective layer 25 in the top R emission region so that the pixeldriver does not (or the TFTs do not) affect a bottom emission operation.

As described above, the top first electrode 30 and the top reflectivelayer 25 are formed in the top R emission region. Thus, the top firstelectrode 30 formed with the top reflective layer 25 can be a topemission electrode to increase (or maximize) the amount of light emittedtoward a top surface. The bottom first electrode 31 formed in the bottomR emission region can be a bottom emission electrode.

In addition, a pixel defining layer 40 is formed on portions of the topfirst electrode 30 and the bottom first electrode 31 to expose otherportions of the top and bottom first electrodes 30 and 31.

An organic layer 50 including at least an organic emission layer (EML)is formed on the exposed portions of the top and bottom first electrodes30 and 31. The organic layer 50 may include not only the EML but alsoone or more layers selected from the group consisting of a holeinjection layer (HIL), a hole transport layer (HTL), an electrontransport layer (ETL), and an electron injection layer (EIL).

The organic layer 50 is deposited by a vacuum evaporation method, a spincoating method, an inkjet printing method, a Doctor blade method, and alaser induced thermal imaging (LITI) method. In addition, the organiclayer 50 may be patterned in accordance with red (R), green (G), andblue (B) unit pixels (or sub-pixels) of a display region. The patterningof the organic layer 50 may be performed by an LITI method or a vacuumevaporation method using a shadow mask.

Thereafter, a second electrode 60 is formed on the entire surface of theglass substrate 10 including the organic layer 50. Here, when the topfirst electrode 30 and the bottom first electrode 31 are anodes, thesecond electrode 60 may be a transmissive electrode formed of aconductive metal having a low work function, i.e., a material selectedfrom the group consisting of Mg, Ca, Al, Ag, and alloys thereof. Whenthe top first electrode 30 and the bottom first electrode 31 arecathodes, the second electrode 60 may be formed of a transparentelectrode such as ITO or IZO. In one embodiment, the second electrode 60may be formed of an alloy of Mg and Ag. The second electrode 60 may beformed by a vacuum evaporation method or a sputtering process.

A bottom reflective layer 70 is also patterned in the top R emissionregion. The bottom reflective layer 70 is formed of Al(Nd) or Ag,preferably, Al(Nd). The bottom reflective layer 70 is formed by shiftingfrom the R pixel region to a G pixel region and from the G pixel regionto a B pixel region using a fine metal mask (FMM).

Subsequently, the glass substrate 10 including the bottom reflectivelayer 70 is attached to an encapsulation substrate, thereby completingthe dual emission R OLED.

As described above, in the present embodiment, the LTPS process offorming the top reflective layer 25 further includes one mask process(e.g., the etch mask process), and the bottom reflective layer 70 isformed using the FMM, so that a dual emission organic light emittingdisplay device can be realized to display different images on bothsurfaces.

Also, the dual emission organic light emitting display device accordingto the present embodiment adopts a time division control (TDC) mode inorder to display different images on top and bottom surfaces using onepixel driving circuit.

Hereinafter, a pixel circuit for the dual emission organic lightemitting display device according to an embodiment of the presentembodiment and a method of driving the same will be described in moredetail with reference to FIGS. 2 and 3.

FIG. 2 is a circuit diagram of R, G, and B pixel circuits of the dualemission organic light emitting display device of FIG. 1.

Referring to FIG. 2, one pixel circuit is divided into an R pixelcircuit 100R, a G pixel circuit 100G, and a B pixel circuit 100B.

The R pixel circuit 100R includes a pixel driver 110R, a top/bottomselector 130R, and an R OLED 150R.

The pixel driver 110R is enabled in response to a previous scan signalSCAN[n-1] and a current scan signal SCAN[n]. Also, the pixel driver 110Rreceives a top or bottom R data signal DATA_R[m] from a data line 111Rand generates a top driving current or a bottom driving current.

The top/bottom selector 130R allows or cuts off the flow of the top orbottom driving current in response to a top emission control signalEMC1[n] or a bottom emission control signal EMC2[n].

The R OLED 150R is divided into a top R OLED and a bottom R OLED. Thetop R OLED receives the top driving current selected by the top/bottomselector 130R and emits R light to the top surface, and the bottom ROLED receives the bottom driving current selected by the top/bottomselector 130R and emits R light to the bottom surface.

That is, in each pixel circuit, one frame is divided into a 1^(st)sub-frame and a 2^(nd) sub-frame. Thus, the top emission data signal isapplied to the pixel driver 110R for the 1^(st) sub-frame such that thetop R OLED emits light, and the bottom emission data signal is appliedto the pixel driver 110R for the 2^(nd) sub-frame such that the bottom ROLED emits light. Accordingly, one pixel circuit is driven using a TDCmethod so that different images can be displayed on the top and bottomsurfaces for one frame.

The components of the R pixel circuit 100R will now be described in moredetail. That is, as shown in FIG. 2, the pixel driver 110R includes fourtransistors M1, M2, MS, and MD and two capacitors Cst and Cvth. That is,a switching transistor MS is connected between a data line 111R and anode N1, and a current scan line 115 is connected to a gate terminal ofthe switching transistor MS. Thus, the switching transistor MS is turnedon in response to the current scan signal SCAN[n] transmitted from thecurrent scan line 115 and transmits the data signal DATA_R[m].

A driving transistor MD has a first electrode connected to a positivepower supply voltage line 117 and a gate terminal connected to a nodeN2. The driving transistor MD generates a driving current correspondingto a voltage applied to the gate terminal thereof.

A threshold voltage compensation transistor M1 is connected between thegate terminal and a second electrode of the driving transistor MD andhas a gate terminal connected to a previous scan line 113. The thresholdvoltage compensation transistor M1 is turned on in response to theprevious scan signal SCAN[n-1] so that the driving transistor MD isdiode-connected.

A power supply voltage application transistor M2 is connected betweenthe positive power supply voltage line 117 and the node N1 and has agate terminal connected to the previous scan line 113. The power supplyvoltage application transistor M2 is turned on in response to theprevious scan signal SCAN[n-1] and transmits a positive power supplyvoltage ELVDD to the node N1.

A first capacitor Cst is connected between the positive power supplyvoltage line 117 and the node N1 and stores a voltage corresponding to adifference between the positive power supply voltage ELVDD and a datavoltage.

A second capacitor Cvth is connected between the node N1 and the gateterminal of the driving transistor MD and stores the threshold voltageof the driving transistor MD.

Here, the transistors M1, M2, MS, and MD of the pixel driver 110R areshown as p-type metal oxide semiconductor field effect transistors(MOSFETs) (hereinafter, PMOS transistors or p-type charge carriertransistors). However, the present invention is not thereby limited. Forexample, the pixel driver 110R may be configured with NMOS transistors(or n-type charge carrier transistors). Also, a circuit configuration ofthe pixel driver 110R is not limited to four transistors and twocapacitors, and embodiments of the present invention may have anysuitable configuration that can supply a driving current.

An operation of the pixel driver 110R will now be described in moredetail. Here, when the previous scan signal SCAN[n-1] is at a low level,the threshold voltage compensation transistor M1 and the power supplyvoltage application transistor M2 are turned on. Thus, a positive powersupply voltage ELVDD is applied to the node N1, and the drivingtransistor MD is diode-connected, so that a voltage ofELVDD−|Vth_(MD)|[V] is applied to the node N2. Accordingly, the secondcapacitor Cvth stores the same voltage as a threshold voltage Vth.

Next, when the current scan signal SCAN[n] is at a low level, theswitching transistor MS is turned on and transmits a data voltage Vdatato the node N1. Here, since the second capacitor Cvth retains the samevoltage as the threshold voltage Vth, a voltage of the node N2 (i.e., agate voltage of the driving transistor MD) becomesELVDD−|Vth_(MD)|−(ELVDD−Vdata)[V].

Therefore, a driving current supplied from the driving transistor MD tothe top or bottom R OLED can be expressed by Equation 1:|I _(ROLED) =k(Vsg _(MD) −|Vth _(MD)|)² =k(ELVDD−(Vdata−|Vth_(MD)|)−|Vth _(MD)|)² =k(ELVDD−Vdata)²  (1),

where k is a constant, ELVDD is the positive power supply voltage, Vdatais a gray level data voltage, and Vth_(MD) is the threshold voltage ofthe driving transistor MD.

Referring still to FIG. 2, the top/bottom selector 130R includes twotransistors MT and MB. That is, a top emission transistor MT isconnected between the second electrode of the driving transistor MD andthe anode of the top R OLED and has a gate terminal connected to a topemission control line 131. The top emission transistor MT is turnedon/off in response to the top emission control signal EMC1[n] and allowsor cuts off the flow of the driving current supplied from the drivingtransistor MD.

A bottom emission transistor MB is connected between the secondelectrode of the driving transistor MD and the anode of the bottom ROLED and has a gate terminal connected to a bottom emission control line132. The bottom emission transistor MB is turned on/off in response tothe bottom emission control signal EMC2[n] and allows or cuts off theflow of the driving current supplied from the driving transistor MD.

Here, the on/off operations of the top emission transistor MT and thebottom emission transistor MB are complementary to each other. In otherwords, the top emission transistor MT is turned on when the bottomemission transistor MB is turned off for the top emission frame of oneframe, so that the top R OLED emits light. Similarly, the top emissiontransistor MT is turned off when the bottom emission transistor MB isturned on for the bottom emission frame of the one frame, so that thebottom R OLED emits light.

Here, the top and bottom emission transistors MT and MB of thetop/bottom selector 130R are PMOS transistors.

Also, although the top/bottom selector 130R is shown to include two PMOStransistors in the present embodiment, the present invention is notthereby limited. For example, the top/bottom selector 130R may includetwo NMOS transistors, or a PMOS transistor and an NMOS transistor.

Referring still to FIG. 2, the R OLED 150R includes the top R OLED andthe bottom R OLED.

That is, in one embodiment, as described with reference to FIG. 1, thetop R OLED includes the top first electrode 30, which is an anode formedwith the top reflective layer 25, the organic layer 50, and the secondelectrode 60, which is a cathode. Here, the anode (or the top firstelectrode) 30 of the top R OLED is connected to a second electrode ofthe top emission transistor MT, and the cathode (or the secondelectrode) 60 is connected to a negative power supply voltage line(e.g., a reference or ground power supply voltage line) 151.

Also, in one embodiment, as described with reference to FIG. 1, thebottom R OLED includes the bottom first electrode 31, which is an anode,the organic layer 50, and the second electrode 60, which is a cathodeformed with the bottom reflective layer 70. The anode (or the bottomfirst electrode) 31 of the bottom R OLED is connected to a secondelectrode of the bottom emission transistor MB, and the cathode (or thesecond electrode) 60 is connected to the negative power supply voltageline 151.

Accordingly, a top R OLED and a bottom R OLED have an R organic emissionlayer (e.g., the organic layer 50) in common and time-divide one frameso that different images can be displayed on the top surface and thebottom surface.

As described above, the dual emission organic light emitting displaydevice of the present embodiment includes a single pixel driver and asingle OLED (e.g., having a top OLED and a bottom OLED with a sharedorganic layer) and drives the OLED using a TDC method, and thusdifferent images can be displayed on the top and bottom surfaces for oneframe.

A method of sequentially driving the top and bottom surfaces of theabove-described organic light emitting display device using a TDC methodwill now be described in more detail with reference to FIG. 3.

FIG. 3 is a timing diagram illustrating a method of driving the pixelcircuit of FIG. 2.

One frame is divided into a 1^(st) sub-frame and a 2^(nd) sub-frame, andit will be assumed that the 1^(st) sub-frame is a top emission sub-frameand the 2^(nd) sub-frame is a bottom emission sub-frame, but the presentinvention is not thereby limited. For example, the 1^(st) sub-frame maybe a bottom emission sub-frame and the 2^(nd) sub-frame may be a topemission sub-frame.

Referring to FIG. 3, for the 1^(st) sub-frame of one frame, a first scansignal SCAN[1] and first top R, G, and B data signals are applied to R,G, and B pixel drivers 110R, 110G, and 110B in a first row of a displayregion having red (R), green (G), and blue (B) unit pixels (orsub-pixels). Thus, the R, G, and B pixel drivers 110R, 110G, and 110Bgenerate a first top R driving current, a first top G driving current,and a first top B driving current, respectively.

In this case, a first top emission control signal EMC1[1] of a low levelis applied through a top emission control line 131 to R, G, and Btop/bottom selectors 130R, 130G, and 130B, and a second bottom emissioncontrol signal EMC2[1] of a high level is applied through a bottomemission control line 132 to the R, G, and B top/bottom selectors 130R,130G, and 130B. Thus, the top emission transistor MT is turned on, andthe bottom emission transistor MB is turned off. Also, the first top R,G, and B driving currents are supplied through the top emissiontransistor MT to the top R OLED, top G OLED, and top B OLED,respectively, so that the top surface of the dual emission organic lightemitting display device emits light with a certain (or predetermined)luminance.

Next, for the 2^(nd) sub-frame of the one frame, another first scansignal SCAN[1] and first bottom R, G, and B data signals are applied tothe R, G, and B pixel drivers 110R, 110G, and 110B in the first row ofthe display region. Thus, the R, G, and B pixel drivers 110R, 110G, and110B generate a first bottom R driving current, a first bottom G drivingcurrent, and a first bottom B driving current, respectively.

In this case, the first top emission control signal EMC1[1] of a highlevel is applied through the top emission control line 131 to the R, G,and B top/bottom selectors 130R, 130G, and 130B, and the second bottomemission control signal EMC2[1] of a low level is applied through thebottom emission control line 132 to the R, G, and B top/bottom selectors130R, 130G, and 130B. Thus, the top emission transistor MT is turnedoff, and the bottom emission transistor MB is turned on. Also, the firstbottom R, G, and B driving currents are supplied through the bottomemission transistor MB to the bottom R OLED, bottom G OLED, and bottom BOLED, respectively, so that the bottom surface of the dual emissionorganic light emitting display device emits light with a certain (orpredetermined) luminance.

Thereafter, for each sub-frame of one frame, a second scan signalSCAN[2] and second top and bottom R, G, and B data signals are appliedto R, G, and B pixel drivers 110R, 110G, and 110B in a second row of thedisplay region. Thus, the top emission transistors MT of the top/bottomselectors 130R, 130G, and 130B are turned on for the 1^(st) sub-frame,so that top R, G, and B OLEDs in the second row emit light. Also, thebottom emission transistors MB of the top/bottom selectors 130R, 130G,and 130B are turned on for the 2^(nd) sub-frame, so that bottom R, G,and B OLEDs in the second row emit light.

The above-described operations are repeated so that an n-th scan signalSCAN[n] and m-th top and bottom R, G, and B data signals are applied toR, G, and B pixel drivers 110R, 110G, and 110B in an n-th row of thedisplay region for each sub-frame of one frame. Thus, the top emissiontransistors MT of the top/bottom selectors 130R, 130G, and 130B areturned on for the 1^(st) sub-frame, so that top R, G, and B OLEDs in then-th row emit light. Also, the bottom emission transistors MB of thetop/bottom selectors 130R, 130G, and 130B are turned on for the 2^(nd)sub-frame, so that bottom R, G, and B OLEDs in the n-th row emit light.

As described above, one frame is divided into two sub-frames (i.e., thetop emission frame and the bottom emission frame), and the top OLED andthe bottom OLED are time-divided for the two sub-frames of the one frameand are sequentially driven so that different images are displayed onthe top and bottom surfaces.

In this case, although the top and bottom OLEDs are time-divided andsequentially driven, since the top and bottom OLEDs are sequentiallydriven in a very short amount of time, a user perceives the top andbottom OLEDs to emit light at the same time. Therefore, different imagescan be independently and normally displayed on the top and bottomsurfaces.

In the present embodiment, one OLED is divided into the top OLED and thebottom OLED, which have a pixel driver in common and are driven using aTDC method. As a result, the circuit configuration of the pixel driveris reduced by half, and thus the dual emission organic light emittingdisplay device can be improved in terms of a layout, an interconnection,and an aperture ratio.

Hereinafter, another pixel circuit of a dual emission organic lightemitting display device that displays the same image on a top surfaceand a bottom surface in accordance with another embodiment of thepresent invention and a method of driving the same will be described.

Embodiment 2

FIG. 4 is a circuit diagram of R, G, and B pixel circuits of a dualemission organic light emitting display device according to anotherembodiment of the present invention.

Referring to FIG. 4, one pixel circuit is divided into an R pixelcircuit 200R, a G pixel circuit 200G, and a B pixel circuit 200B. Here,only the R pixel circuit 200R will be described in detail for ease ofexplanation.

The R pixel circuit 200R includes a pixel driver 210R, a top/bottomselector 230R, and an R OLED 250R.

The pixel driver 210R is enabled in response to a previous scan signalSCAN[n-1] and a current scan signal SCAN[n], receives an R data signalDATA_R[m] from a data line 211R, and generates a driving current. Also,the pixel driver 210R receives an emission control signal EMI[n] andallows or cuts off the flow of the driving current in response to theemission control signal EMI[n].

The top/bottom selector 230R allows the driving current to flow into atop R OLED or a bottom R OLED in response to a top/bottom selectionsignal DIR. Here, the top/bottom selection signal DIR is commonlyapplied to top/bottom selectors of all pixels.

Here, the R OLED 250R is divided into the top R OLED and the bottom ROLED. The top R OLED receives a top driving current selected by thetop/bottom selector 230R and emits R light through a top surface, andthe bottom R OLED receives a bottom driving current selected by thetop/bottom selector 230R and emits R light through a bottom surface.

In the present embodiment, each pixel circuit of the dual emissionorganic light emitting display device may emit light through only one ofthe top surface or the bottom surface in response to the top/bottomselection signal DIR. In other words, an emission surface can beselectively determined, if required. Also, each pixel circuit includes apixel driver and an OLED and applies the top/bottom selection signal DIRas a pulse type signal so that the same image can be displayed on boththe top and bottom surfaces.

The components of the pixel driver 210R will now be described in moredetail. The pixel driver 210R includes five transistors M1, M2, MS, MD,and ME and two capacitors Cst and Cvth. Specifically, a switchingtransistor MS is connected between the data line 211R and a node N1 andhas a gate terminal connected to a current scan line 215. Thus, theswitching transistor MS is turned on in response to a current scansignal SCAN[n] supplied from the current scan line 215 and transmits theR data signal DATA_R[m].

A driving transistor MD has a first electrode connected to a positivepower supply voltage line 217 and a gate terminal connected to a nodeN2. The driving transistor MD generates a driving current correspondingto a voltage applied to the gate terminal thereof.

A threshold voltage compensation transistor M1 is connected between thegate terminal and a second electrode of the driving transistor MD andhas a gate terminal connected to the previous scan line 213. Thethreshold voltage compensation transistor M1 is turned on in response tothe previous scan signal SCAN[n-1] so that the driving transistor MD isdiode-connected.

A power supply voltage application transistor M2 is connected betweenthe positive power supply voltage line 217 and the node N1 and has agate terminal connected to the previous scan line 213. The power supplyvoltage application transistor M2 is turned on in response to theprevious scan signal SCAN[n-1] and transmits a positive power supplyvoltage ELVDD to the node N1.

A first capacitor Cst is connected between the positive power supplyvoltage line 217 and the node N1 and stores a voltage corresponding to adifference between the positive power supply voltage ELVDD and a datavoltage.

A second capacitor Cvth is connected between the node N1 and the gateterminal of the driving transistor MD and stores the threshold voltageof the driving transistor MD.

An emission control transistor ME is connected between the secondelectrode of the driving transistor MD and the top/bottom selector 230Rand has a gate terminal connected to the emission control line 217. Theemission control transistor ME allows or cuts off the flow of thedriving current in response to the emission control signal EMI[n].

The transistors M1, M2, MS, MD, and ME of the pixel driver 210R areshown as PMOS transistors. However, the present invention is not therebylimited. For example, the pixel driver 210R may be configured with NMOStransistors. Also, a circuit configuration of the pixel driver 210R isnot limited to five transistors and two capacitors, and embodiments ofthe present invention may have any suitable configuration that cansupply a driving current.

An operation of the pixel driver 210R will now be described in moredetail. Here, when the previous scan signal SCAN[n-1] of a low level isapplied, the threshold voltage compensation transistor M1 and the powersupply voltage application transistor M2 are turned on. Thus, a positivepower supply voltage ELVDD is applied to the node N1, and the drivingtransistor MD is diode-connected, so that a voltage ofELVDD−|Vth_(MD)|[V] is applied to the node N2. Accordingly, the secondcapacitor Cvth stores the same voltage as a threshold voltage Vth.

Next, when the current scan signal SCAN[n] of a low level is applied,the switching transistor MS is turned on and transmits a data voltageVdata to the node N1. Here, since the second capacitor Cvth retains thesame voltage as the threshold voltage Vth, a voltage at the node N2(i.e., a gate voltage of the driving transistor MD) becomesELVDD−|Vth_(MD)|−(ELVDD−Vdata)[V].

Then, when the emission control signal EMI[n] of a low level is applied,the emission control transistor ME is turned on and allows the flow of adriving current corresponding to a voltage applied to the gate terminalof the driving transistor MD.

Therefore, a driving current supplied from the driving transistor MDthrough the emission control transistor ME to the R OLED can beexpressed by the above-described Equation 1.

Referring still to FIG. 4, the top/bottom selector 230R includes twotransistors MT′ and MB′. That is, a top emission transistor MT′ isconnected between a second electrode of the emission control transistorME and an anode of the top R OLED and has a gate terminal connected to atop/bottom selection line 231. The top emission transistor MT′ is turnedon/off in response to the top/bottom selection signal DIR and allows orcuts off the flow of the driving current.

A bottom emission transistor MB′ is connected between the secondelectrode of the emission control transistor ME and an anode of thebottom R OLED and has a gate terminal connected to the top/bottomselection line 231. The bottom emission transistor MB′ is turned on/offin response to the top/bottom selection signal DIR and allows or cutsoff the flow of the driving current.

Here, gate terminals of the top and bottom emission transistors MT′ andMB′ are commonly connected to the top/bottom selection line 231 so thatthe on/off operations of the top emission transistor MT′ and the bottomemission transistor MB′ can be complementary to each other. Although itis illustrated in FIG. 4 that the top emission transistor MT′ is a PMOStransistor and the bottom emission transistor MB′ is an NMOS transistor,the present invention is not thereby limited. For example, the topemission transistor MT′ may be an NMOS transistor and the bottomemission transistor MB′ may be a PMOS transistor.

Referring still to FIG. 4, the R OLED 250R includes the top R OLED andthe bottom R OLED.

That is, in one embodiment, as described with reference to FIG. 1, thetop R OLED includes the top first electrode 30, which is an anode formedwith the top reflective layer 25, the organic layer 50, and the secondelectrode 60, which is a cathode. Here, the anode 30 (or the top firstelectrode) 30 of the top R OLED is connected to a second electrode ofthe top emission transistor MT′, and the cathode (or the secondelectrode) 60 is connected to a negative power supply voltage line(e.g., a reference or ground power supply voltage line) 251.

Also, in one embodiment, as described with reference to FIG. 1, thebottom R OLED includes the bottom first electrode 31, which is an anode,the organic layer 50, and the second electrode 60, which is a cathodeformed with the bottom reflective layer 70. The anode (or the bottomfirst electrode) 31 of the bottom R OLED is connected to a secondelectrode of the bottom emission transistor MB′, and the cathode (or thesecond electrode) 60 is connected to the negative power supply voltageline 251.

As described above, the dual emission organic light emitting displaydevice of the present embodiment includes a single pixel driver and asingle OLED (e.g., having a top OLED and a bottom OLED with a sharedorganic layer) and can emit light through only one of the top surface orthe bottom surface in response to a top/bottom selection signal, ifdesired. Also, the dual emission organic light emitting display deviceof the present embodiment can apply the top/bottom selection signal as apulse type signal so that the same image can be displayed on both thetop and bottom surfaces.

A method of driving the above-described organic light emitting displaydevice will now be described in more detail with reference to FIGS. 5and 6.

FIG. 5 is a timing diagram illustrating a first method of driving thepixel circuit of FIG. 4.

Referring to FIG. 5, for a 1^(st) frame, scan signals SCAN[1] to SCAN[n]and emission control signals EMI[1] to EMI[n] are sequentially appliedto pixel drivers 210R, 210G, and 210B, and data signals DATA_R[1] toDATA_R[m] are applied to corresponding pixels, and thus a drivingcurrent (which may be predetermined) flows through the pixel drivers210R, 210G, and 210B.

In this case, a top/bottom selection signal DIR that is commonly appliedto top/bottom selectors 230R, 230G, and 230B is dropped to a low level.Thus, the top emission transistor MT′ is turned on, and the bottomemission transistor MB′ is turned off, so that the driving current flowsinto top OLEDs to display an image on the top surface.

Next, for a 2^(nd) frame, the scan signals SCAN[1] to SCAN[n] and theemission control signals EMI[1] to EMI[n] are sequentially applied tothe pixel drivers 210R, 210G, and 210B, and the data signals DATA_R[1]to DATA_R[m] are applied to the corresponding pixels, and thus a drivingcurrent (which may be predetermined) flows through the pixel drivers210R, 210G, and 210B.

In this case, the top/bottom selection signal DIR that is commonlyapplied to the top/bottom selectors 230R, 230G, and 230B is elevated toa high level. Thus, the top emission transistor MT′ is turned off, andthe bottom emission transistor MB′ is turned on, so that the drivingcurrent flows into bottom OLEDs to display an image on the bottomsurface.

As described above, the pixel circuit of the dual emission organic lightemitting display device of the present embodiment includes a singlepixel driver and an OLED so that the same image can be displayed on thetop surface or the bottom surface, if desired, using the top/bottomselector.

FIG. 6 is a timing diagram illustrating a second method of driving thepixel circuit of FIG. 4.

Referring to FIG. 6, for each frame, scan signals SCAN[1] to SCAN[n] andemission control signals EMI[1] to EMI[n] are sequentially applied tothe pixel drivers 210R, 210G, and 210B, and data signals DATA_R[1] toDATA_R[m] are applied to corresponding pixels, and thus a drivingcurrent (which may be predetermined) flows through the pixel drivers210R, 210G, and 210B.

In this case, a pulse-type top/bottom selection signal DIR, whichalternates between a low level and a high level in a certain orpredetermined cycle, is commonly applied to the top/bottom selectors230R, 230G, and 230B. The pulse-type top/bottom selection signal DIR hasa duty ratio of 50%. This leads the top and bottom surfaces to emitlight for the same time, thus making luminance uniform. Accordingly, thetop and bottom emission transistors MT′ and MB′ are repeatedly andalternately turned on/off in response to the top/bottom selection signalDIR to repeatedly and alternately flow the driving current into the topand bottom OLEDs so that the same image can be displayed on the top andbottom surfaces.

As described above, the pixel circuit of the dual emission organic lightemitting display device of the present embodiment includes a singlepixel driver and a single OLED and can display the same image on boththe top and bottom surfaces in a time-division manner using thepulse-type top/bottom selection signal. In this case, since the top andbottom OLEDs are sequentially driven in a very short amount of time, auser perceives the top and bottom OLEDs to emit light at the same time.Therefore, the same image can be normally displayed on both the top andbottom surfaces.

In the present embodiment, one OLED is divided into the top OLED and thebottom OLED, which have a pixel driver in common. Thus, an image isdisplayed on only one of the top surface or the bottom surface, ifdesired, by controlling a top/bottom selection signal. Alternatively,the same image can be displayed in a time-division manner on both thetop and bottom surfaces using a pulse-type top/bottom selection signal.As a result, the circuit configuration of the pixel driver is reduced byhalf, and thus the dual emission organic light emitting display devicecan be improved in terms of a layout, an interconnection, and anaperture ratio.

According to an embodiment of the present invention as described above,a single OLED is divided into a top OLED and a bottom OLED, which have apixel driver in common. Thus, the circuit configuration of the pixeldriver is reduced by half so that a dual emission organic light emittingdisplay device can be improved in terms of a layout, an interconnection,and an aperture ratio.

Also, since the dual emission organic light emitting display device canbe driven by a TDC method by dividing one frame into a top emissionframe and a bottom emission frame, different images can be displayed ona top surface and a bottom surface.

Further, the dual emission organic light emitting display device cancontrol a top/bottom selection signal to display an image on one of thetop surface or the bottom surface, if desired. Alternatively, the dualemission organic light emitting display device can apply the top/bottomselection signal as a pulse type signal so that the same image can bedisplayed in a time-division manner on both the top and bottom surfaces.

While the invention has been described in connection with certainexemplary embodiments, it is to be understood by those skilled in theart that the invention is not limited to the disclosed embodiments, but,on the contrary, is intended to cover various modifications includedwithin the spirit and scope of the appended claims and equivalentsthereof.

1. A dual emission organic light emitting display device comprising: apixel driver adapted to receive a top emission data signal for a firstsub-frame of one frame, to receive a bottom emission data signal for asecond sub-frame of the one frame, and to generate a top driving currentand a bottom driving current; a top/bottom selector adapted to receivethe top driving current and the bottom driving current from the pixeldriver and to selectively supply the top driving current and the bottomdriving current in response to a top emission control signal and abottom emission control signal; and a dual emission organic lightemitting diode adapted to receive one of the top driving current or thebottom driving current, to emit light through a top surface for thefirst sub-frame, and to emit light through a bottom surface for thesecond sub-frame.
 2. The dual emission organic light emitting displaydevice according to claim 1, wherein the top/bottom selector comprises:a top emission transistor connected between the pixel driver and a topanode of the dual emission organic light emitting diode and adapted toturn on or off in response to the top emission control signal; and abottom emission transistor connected between the pixel driver and abottom anode of the dual emission organic light emitting diode andadapted to turn on or off in response to the bottom emission controlsignal.
 3. The dual emission organic light emitting display deviceaccording to claim 2, wherein the top emission transistor is turned onfor the first sub-frame to supply the top driving current, and whereinthe bottom emission transistor is turned on for the second sub-frame tosupply the bottom driving current.
 4. The dual emission organic lightemitting display device according to claim 3, wherein the top and bottomemission transistors are MOS transistors having the same charge carriertype.
 5. The dual emission organic light emitting display deviceaccording to claim 4, wherein the dual emission organic light emittingdiode displays different images on the top surface and the bottomsurface.
 6. The dual emission organic light emitting display deviceaccording to claim 5, wherein the pixel driver comprises: a switchingtransistor connected between a data line and a first node and adapted toapply one of the top emission data signal or the bottom emission datasignal in response to a current scan signal transmitted through a gateterminal thereof; a driving transistor connected between a first powersupply voltage line and the top/bottom selector and adapted to generateone of the top driving current or the bottom driving current inaccordance with a voltage at a second node connected to a gate terminalthereof; a threshold voltage compensation transistor connected betweenthe gate terminal and a drain terminal of the driving transistor andadapted to diode-connect the driving transistor in response to aprevious scan signal; a power supply voltage application transistorconnected between the first power supply voltage line and the first nodeand adapted to apply a first power supply voltage to the first node inresponse to the previous scan signal; a first capacitor connectedbetween the first power supply voltage line and the first node andadapted to store a voltage corresponding to a difference between thefirst power supply voltage and a voltage of the one of the top emissiondata signal or the bottom emission data signal; and a second capacitorconnected between the first node and the gate terminal of the drivingtransistor and adapted to store a voltage corresponding to a thresholdvoltage of the driving transistor.
 7. A dual emission organic lightemitting display device comprising: a pixel driver adapted to generate adriving current in response to a data signal; a top/bottom selectoradapted to receive the driving current from the pixel driver and toselectively supply the driving current in response to a top/bottomselection signal; and a dual emission organic light emitting diodeadapted to selectively receive the driving current from the top/bottomselector and to emit light through one of a top surface or a bottomsurface.
 8. The dual emission organic light emitting display deviceaccording to claim 7, wherein the top/bottom selector comprises: a topemission transistor connected between the pixel driver and a top anodeof the dual emission organic light emitting diode and adapted to turn onor off in response to the top/bottom selection signal; and a bottomemission transistor connected between the pixel driver and a bottomanode of the dual emission organic light emitting diode and adapted toturn on or off in response to the top/bottom selection signal.
 9. Thedual emission organic light emitting display device according to claim8, wherein the top and bottom emission transistors are MOS transistorshaving different charge carrier types, and wherein the top and bottomemission transistors are adapted to perform complementary on/offoperations in response to the top/bottom selection signal.
 10. The dualemission organic light emitting display device according to claim 9,wherein the top/bottom selection signal is commonly applied to thetop/bottom selectors of all pixels of the dual emission organic lightemitting display device.
 11. The dual emission organic light emittingdisplay device according to claim 10, wherein the top/bottom selectionsignal is a pulse-type signal with a certain cycle, and wherein thepulse-type signal has a duty ratio of 50%.
 12. The dual emission organiclight emitting display device according to claim 11, wherein the dualemission organic light emitting diode displays the same image on boththe top and bottom surfaces.
 13. The dual emission organic lightemitting display device according to claim 12, wherein the pixel drivercomprises: a switching transistor connected between a data line and afirst node and adapted to apply the data signal in response to a currentscan signal transmitted through a gate terminal thereof; a drivingtransistor connected between a first power supply voltage line and thetop/bottom selector and adapted to generate the driving current inaccordance with a voltage at a second node connected to a gate terminalthereof; a threshold voltage compensation transistor connected betweenthe gate terminal and a drain terminal of the driving transistor andadapted to diode-connect the driving transistor in response to aprevious scan signal; a power supply voltage application transistorconnected between the first power supply voltage line and the first nodeand adapted to apply a first power supply voltage to the first node inresponse to the previous scan signal; a first capacitor connectedbetween the first power supply voltage line and the first node andadapted to store a voltage corresponding to a difference between thefirst power supply voltage and a voltage of the data signal; a secondcapacitor connected between the first node and the gate terminal of thedriving transistor and adapted to store a voltage corresponding to athreshold voltage of the driving transistor; and an emission controltransistor connected between the drain terminal of the drivingtransistor and the top/bottom selector and adapted to allow or cut offthe flow of the driving current in response to an emission controlsignal transmitted through a gate terminal thereof.
 14. A dual emissionorganic light emitting display device comprising: a top organic lightemitting diode having a top first electrode formed with a top reflectivelayer, an organic layer, and a second electrode, the organic layer beingstacked between the top first electrode and the second electrode; abottom organic light emitting diode having a bottom first electrode, theorganic layer, and a second electrode formed with a bottom reflectivelayer, the organic layer being stacked also between the bottom firstelectrode and the second electrode; and a pixel driver electricallyconnected to both the top first electrode and the bottom first electrodeand adapted to selectively supply a driving current to the top firstelectrode and the bottom first electrode.
 15. The dual emission organiclight emitting display device according to claim 14, wherein the pixeldriver is disposed under the top reflective layer and formed in a topemission region.
 16. The dual emission organic light emitting displaydevice according to claim 14, wherein the pixel driver comprises aplurality of TFTs adapted to selectively supply the driving current tothe top first electrode and the bottom first electrode.
 17. The dualemission organic light emitting display device according to claim 16,wherein the plurality of TFTs are formed under the top reflective layerin the top emission region so that the TFTs do not affect a bottomemission operation
 18. The dual emission organic light emitting displaydevice according to claim 14, wherein the top first electrode and thebottom first electrode are transparent electrodes formed of at least onematerial selected from the group consisting of indium tin oxide (ITO)and indium zinc oxide (IZO).
 19. The dual emission organic lightemitting display device according to claim 14, wherein the secondelectrode is a transmissive electrode formed of at least one materialselected from the group consisting of Mg, Ca, Al, Ag, and alloysthereof.
 20. The dual emission organic light emitting display deviceaccording to claim 14, wherein the bottom reflective layer is formed byshifting from a red pixel region to a green pixel region and from thegreen pixel region to a blue pixel region using a fine metal mask.